QuickOPC User's Guide and Reference
ReadItem<TValue>(IEasyDAClient,String,String,String) Method



OpcLabs.EasyOpcClassicCore Assembly > OpcLabs.EasyOpc.DataAccess.Extensions Namespace > IEasyDAClientExtension2 Class > ReadItem Method : ReadItem<TValue>(IEasyDAClient,String,String,String) Method
The type of item value.
The client object that will perform the operation.
Name of the machine. Determines the computer on which the OPC server is located. May be an empty string, in which case the OPC server is assumed to exist on the local computer or at the computer specified for it by DCOM configuration.
Contains ProgID of the OPC server to read from.
Contains OPC item identifier.
Reads a typed named item from an OPC server. A typed value, quality and timestamp are returned. Reads a typed named item using individual parameters specifying its location, and a specific data type.
Syntax
'Declaration
 
<ExtensionAttribute()>
<NotNullAttribute()>
Public Overloads Shared Function ReadItem(Of TValue)( _
   ByVal client As IEasyDAClient, _
   ByVal machineName As String, _
   ByVal serverClass As String, _
   ByVal itemId As String _
) As DAVtq(Of TValue)
'Usage
 
Dim client As IEasyDAClient
Dim machineName As String
Dim serverClass As String
Dim itemId As String
Dim value As DAVtq(Of TValue)
 
value = IEasyDAClientExtension2.ReadItem(Of TValue)(client, machineName, serverClass, itemId)
[Extension()]
[NotNull()]
public:
static DAVtq<TValue^>^ ReadItemgeneric<typename TValue>
( 
   IEasyDAClient^ client,
   String^ machineName,
   String^ serverClass,
   String^ itemId
) 

Parameters

client
The client object that will perform the operation.
machineName
Name of the machine. Determines the computer on which the OPC server is located. May be an empty string, in which case the OPC server is assumed to exist on the local computer or at the computer specified for it by DCOM configuration.
serverClass
Contains ProgID of the OPC server to read from.
itemId
Contains OPC item identifier.

Type Parameters

TValue
The type of item value.

Return Value

If successful, the function returns a OpcLabs.EasyOpc.DataAccess.Generic.DAVtq<TValue> object. The object contains the value, timestamp and quality of the OPC item requested.
Exceptions
ExceptionDescription

A null reference (Nothing in Visual Basic) is passed to a method that does not accept it as a valid argument.

This is a usage error, i.e. it will never occur (the exception will not be thrown) in a correctly written program. Your code should not catch this exception.

The OPC "Classic" (or OPC XML-DA) operation has failed. This operation exception in uniformly used to allow common handling of various kinds of errors. The System.Exception.InnerException always contains information about the actual error cause.

This is an operation error that depends on factors external to your program, and thus cannot be always avoided. Your code must handle it appropriately.

Remarks

The server can be local or can be remotely accessed via DCOM. Optionally, an access path can be specified or a specific data type can be requested.

The ReadItem method only waits for the first update from the server (or until the timeout elapses); it does not wait until the quality becomes "uncertain" or "good".

Requirements

Target Platforms: .NET Framework: Windows 10 (selected versions), Windows 11 (selected versions), Windows Server 2016, Windows Server 2022; .NET: Linux, macOS, Microsoft Windows

See Also